At present, the high-speed steel rolls that people call are all high-carbon high-speed steel composite rolls, that is, the working layer of the roll is made of high-carbon high-speed steel, and the core material of the roll is made of nodular cast iron, graphite steel or forged steel, etc., two different materials It is made by centrifugal casting or compounded by CPC process, which is fundamentally different from traditional M2, M4 and other standard tungsten-molybdenum high-speed steels. Under normal rolling conditions, the service life of high-speed steel rolls is more than 3 times that of alloy cast iron rolls.
I.The characteristics of high-speed steel rolls
- High-speed steel rolls have high carbon content and high alloying elements such as vanadium, chromium, tungsten, molybdenum, and niobium. Therefore, the types of carbides in the roll structure are mainly MC type and M2C type, and carbide hardness High, good wear resistance.
- The high-speed steel roll has good thermal stability, and the roll surface has high hardness and good wear resistance at the rolling temperature.
- The high-speed steel roll has good hardenability, and the hardness from the surface of the roll body to the inside of the working layer is almost not reduced, so as to ensure that the roll has the same good wear resistance from the outside to the inside.
- During the use of the roll, under good cooling conditions, a thin and dense oxide film is formed on the surface of the roll body. This uniform, thin and dense oxide film exists for a long time without falling off, making the high-speed steel roll wear-resistant Significantly increased.
- The formation of an oxide film on the surface of the high-speed steel roll during rolling reduces the friction between the rolled material and the roll surface, so slipping is likely to occur during the rolling process, which makes the adjustment of the rolling mill more difficult.
- The high-speed steel material has a large expansion coefficient and good thermal conductivity. It is easy to cause changes in the roll shape during rolling and affect the precision of the rolled material. Therefore, when high-speed steel rolls are used on a strip mill, not only the design of the cooling system must be changed, but also the design of the roll shape must be changed again.
- Since the core of the high-speed steel roll is made of alloy ductile iron, graphite steel or forged steel, the strength of the roll neck is high.
- Because high-speed steel materials have good wear resistance and poor accident resistance, the effect of high-speed steel rolls is not only determined by the quality of the high-speed steel rolls themselves, but also more importantly depends on the use conditions of the rolls and the maintenance of the rolls.
II.Processing of high-speed steel rolls
After the hardness of the roll is increased, the difficulty of the roll processing increases accordingly. For the pass processing of the high-speed steel rolls used in the wire and bar rolling mills, especially the cutting processing of the rolled rebar rolls when they are repeatedly used, because of the intermittent cutting , Its processing is more difficult, so the selection of reasonable processing tools and cutting amount is a prerequisite for determining whether the high-speed steel roll can be used normally on the online bar mill.
In the selection of processing tools, if possible, imported cubic boron nitride (CBN) or imported cemented carbide K01 series, such as HIP from Sandvik, can be used. If it is a rolling screw steel roll that is repeatedly used, a cemented carbide tool must be used for turning, and a cemented carbide tool must be used for turning when the rolled screw steel roll is repeatedly used.
The cutting amount of high-speed steel roll, if it is flat roll outer circle processing, the turning speed is selected
About 10m/min, the cutting depth is 1-3mm, and the feed is 0.2-0.5mm/r; if it is hole type processing or threaded hole repeated turning, the turning speed is about 5m/min, and the cutting depth is 0.1-0.2 mm is ideal.
For the rolling of rebar, ribs need to be milled on the rolling groove. The milling machine can be a flying cutter milling machine, and the milling cutter can be YD500 or YD05 or YTI and other carbide milling cutters.
In short, for high-speed steel rolls, whether it is the turning of the pass or the milling of the thread groove, it is very difficult. The processing time is 3-4 times that of the alloy cast iron roll.
III. Cooling conditions and requirements for high-speed steel rolls of wire and bar mills
During the rolling process, there are three purposes for cooling the roll,-to prevent the roll from overheating and eventually cause peeling; the second is to prevent the roll surface temperature from being too high, resulting in changes in the roll surface structure, affecting the wear resistance of the roll and the surface quality of the rolled material; The third is to extend the time of thermal fatigue crack formation and prevent the propagation of thermal fatigue cracks. The heating of the roll mainly comes from the contact with the rolling piece, but the deformation of the rolling piece also has a certain influence on the heating of the roll. It is generally considered that the temperature of the contact area between the rolling piece and the roll is the average value of the rolling piece temperature and the roll temperature. Therefore, the instantaneous temperature of the roll surface in contact with the rolling piece can reach about 500°C.
In particular, the introduction of endless rolling technology has more stringent requirements on the bearing conditions of the rolls, and how to quickly cool the temperature of the roll surface is extremely important. The ideal cooling system should be a series of nozzles distributed in a ring shape. These nozzles can ensure a continuous and sufficient water flow to the rolling groove. Among these nozzles, the main nozzle near the exit guide should be the main nozzle.
The amount of water discharged is 30% of the total amount of water on the entire rolling trough, and the spray angle is 20-30 degrees. The function of this nozzle is to ensure that the cooling water is quickly sprayed on the part of the rolling trough that has just separated from the rolling piece. This is particularly important . The water pressure is controlled above 0.5Mpa, and the water volume per groove of a single roll is not less than 300L/min, 70%-80% of the water volume should be guaranteed on the outlet side. The cooling water must all be flushed in the rolling groove. Because the cooling condition of the lower roll is worse than that of the upper roll, the amount of water in the lower roll is appropriately more than that of the upper roll. The cooling water should be precipitated and the content of solid particles should be reduced as much as possible. Although the roll water cooling device, cooling method, cooling water volume, water pressure and nozzle angle of each rolling mill are quite different. However, lowering the temperature of the roll surface, maintaining a uniform temperature distribution of the roll cooling, and ensuring the cooling effect of the roll surface are essential for increasing the service life of the roll and improving the surface quality of the rolled material. When the cooling method is similar to the water pressure, increasing the cooling water volume of the roll has a significant effect on reducing the temperature of the roll and reducing the hot cracks on the roll surface.
IV.Handling of accidents caused by high-speed steel rolls
According to statistics, the current consumption of abnormal use of metallurgical rolls in my country will account for about 30% of the total roll consumption.
Normal operation of the machine and reduction of rolling accidents are extremely important to reduce the consumption of rolls. When the wire and bar mill uses high-speed steel rolls and steel jams or piles of steel occur, the water should be stopped immediately, and the upper roll should be quickly lifted to make the rolled material leave the roll surface and at the same time the rolled material accumulated on the lower roll is immediately removed to make The heated rolling groove is slowly cooled in the air. When reproducing after the accident is handled, the groove must be changed, and there must be 2-3 grooves apart. It is absolutely not allowed to let the cooling water wash or flow onto the heated rolling groove. To avoid and cannot change the groove, you must change the roll, wait for it to cool down before using it again on the machine, otherwise the cooling water is poured directly on the overheated rolling groove, the rolling groove will inevitably appear cracks.